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 Post subject: 18" wheel fitment
PostPosted: Thu Jun 29, 2017 3:17 am 
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Vehicle: MY08 SH XT Premium

Posts: 13
Location: Victoria
Simple question

I have a Subaru Forester, SH XT Premium.
Stock wheel size is 17x7 +48 (is this correct?)
Stock tire size is 225/55r17



I want to buy some wheels and tires...
18x8.5 +25
225/40r18


Will I have any troubles fitting the new wheels and tires?
mainly concerning the new wheel width and offset....

Thanks in advance.


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 Post subject: Re: 18" wheel fitment
PostPosted: Thu Jun 29, 2017 4:35 am 
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Vehicle: JDM SF STi Pure White

Posts: 130
Location: Germany
Your speedo is going to be off by 6% and they're gonna poke hard. Hard enough to cause serious rubbage.

I've got a parts evaluation for a set of Team Dynamics Monza R in front of me. They're 18x7.5 +37. The Forester SH is listed with 225/45R18 with no modifications to be done to be allowed to drive on these wheels. Take that with a pinch of salt as these papers are made for the german market, but i think the regulations about wheel rubbage will be the same.

And you might want to go check out willtheyfit.com


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 Post subject: Re: 18" wheel fitment
PostPosted: Thu Jun 29, 2017 5:37 am 
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I Say: Fozzy fozzy fozzy, oi oi oi!
Vehicle: 2011 Forester S-Edition WRB

Posts: 1400
Location: 3015
http://www.willtheyfit.com/index.php?wi ... offset2=25

Tyres illegally small, rims poking but could get a away with it (unless you are low might need guard rolling and guard liner cutting)

You would need a 50 series to keep the tyre legal.

http://www.willtheyfit.com/index.php?wi ... offset2=25

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 Post subject: Re: 18" wheel fitment
PostPosted: Thu Jun 29, 2017 9:32 am 
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Vehicle: MY08 SH XT Premium

Posts: 13
Location: Victoria
Thanks guise, looks like I'm not buying them.

Sent from my 0PJA10 using Tapatalk


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 Post subject: Re: 18" wheel fitment
PostPosted: Thu Jun 29, 2017 11:09 am 
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I Say: Mum, when I grow up, I want to be a pilot! Sorry son, you can't do both!! :D
Vehicle: MY05 XT Luxury AT, white

Posts: 1813
Location: Wynnum West, QLD
Add to that, they're track-width change is illegal too. Scrubbing is illegal, and very bad for handling too (instant infinite spring rate).

Page 19 onwards, is where most the good gen is:
https://www.tmr.qld.gov.au/-/media/Safe ... .pdf?la=en

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viewtopic.php?t=30077


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 Post subject: Re: 18" wheel fitment
PostPosted: Thu Jun 29, 2017 8:06 pm 
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Vehicle: JDM SF STi Pure White

Posts: 130
Location: Germany
craig, could you elaborate on the infinite spring rate thing or point me towards some reading material about it?


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 Post subject: Re: 18" wheel fitment
PostPosted: Sun Jul 02, 2017 8:25 pm 
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Vehicle: MY08 SH XT Premium

Posts: 13
Location: Victoria
craig747400 wrote:
Add to that, they're track-width change is illegal too. Scrubbing is illegal, and very bad for handling too (instant infinite spring rate).

Page 19 onwards, is where most the good gen is:
https://www.tmr.qld.gov.au/-/media/Safe ... .pdf?la=en

track-width?

scrubbing??

I'm fairly cluey but you've got me stumped


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 Post subject: Re: 18" wheel fitment
PostPosted: Mon Jul 03, 2017 10:29 am 
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I Say: Mum, when I grow up, I want to be a pilot! Sorry son, you can't do both!! :D
Vehicle: MY05 XT Luxury AT, white

Posts: 1813
Location: Wynnum West, QLD
Phrewfuf wrote:
craig, could you elaborate on the infinite spring rate thing or point me towards some reading material about it?


Sure!

I know our wagons get called 'shopping trolleys' as a joke, but take a look at a shopping trolley. The wheels have caster, which can explain a bit (for other reasons), and there are no springs.

Spring rate is how much weight it takes to compress the springs a defined amount. Different manufacturers / brands / regions of the World use different units of measurement, but it all means the same thing in the end.

With no springs (or a solid structure), it would take an infinite amount of weight to move the structure 0mm in distance = infinte spring rate. Just simple maths! ;)

So ... when a suspension bottoms-out, the springs no longer act like springs, they act like solid blocks = infinite spring rate.

Edit:
Forgot to add the link. Start from the bottom at Part 1
http://www.motoiq.com/Tech/TheUltimateG ... dling.aspx

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 Post subject: Re: 18" wheel fitment
PostPosted: Mon Jul 03, 2017 11:52 am 
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I Say: Mum, when I grow up, I want to be a pilot! Sorry son, you can't do both!! :D
Vehicle: MY05 XT Luxury AT, white

Posts: 1813
Location: Wynnum West, QLD
BotBot wrote:
craig747400 wrote:
Add to that, they're track-width change is illegal too. Scrubbing is illegal, and very bad for handling too (instant infinite spring rate).

Page 19 onwards, is where most the good gen is:
https://www.tmr.qld.gov.au/-/media/Safe ... .pdf?la=en

track-width?

scrubbing??

I'm fairly cluey but you've got me stumped


.

_________________
His name is 'Steve'; he has just a few mods ... that's all! ;)
viewtopic.php?t=30077


Last edited by craig747400 on Mon Jul 03, 2017 11:53 am, edited 1 time in total.

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 Post subject: Re: 18" wheel fitment
PostPosted: Mon Jul 03, 2017 11:53 am 
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I Say: Mum, when I grow up, I want to be a pilot! Sorry son, you can't do both!! :D
Vehicle: MY05 XT Luxury AT, white

Posts: 1813
Location: Wynnum West, QLD
BotBot wrote:
craig747400 wrote:
Add to that, they're track-width change is illegal too. Scrubbing is illegal, and very bad for handling too (instant infinite spring rate).

Page 19 onwards, is where most the good gen is:
https://www.tmr.qld.gov.au/-/media/Safe ... .pdf?la=en

track-width?

scrubbing??

I'm fairly cluey but you've got me stumped


Track width is measured from the centres of the tyres' contact patch (this confuses many people). Narrowing a vehicle's track-width leads towards a more unstable platform. Going from a Forester rim offset of +48mm to a WRX offset of +55mm reduces the track-width of the vehicle (14mm overall) ... this also confuses a lot of people, due to the +/- nature of the use of offset's units of measurement. It also reduces the scrub radius, which reduces steering feedback and feel (but also reduces the stress / moment arm). Going too wide has the opposite effect.

Some people think putting wider tyres on their car, increases the track-width of their car: it does not ... even though the outside of the tyre sits wider, the centre of the rim / contact patch is unchanged, hence the track-width is unchanged. The tyre is also 'wider' towards the centre of the car an equal amount. More grip from a bigger contact patch, but no change to the track-width, unless the offset is different, or other radical suspension changes have been made.

Scrubbing occurs, any time during the suspension's travel, if the tyre comes into contact with any part of the vehicle. There's good reason it's illegal: it's dangerous, for more than one reason.

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His name is 'Steve'; he has just a few mods ... that's all! ;)
viewtopic.php?t=30077


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